The Late Mississippian Bayport Limestone contains a breccia, 1-5 ft thick, between undisturbed, nearly horizontal beds of marine limestone. Fractures and spaces between limestone fragments are filled with crystalline pyrite and calcite. The breccia appears to be the result of breaking by shearing and dislocating movement, but apparently was formed without disturbing strata above or below.

Hypotheses such as tectonic movement or hydrothermal or volcanic intrusion have been rejected because of field evidence. The main sedimentary hypotheses considered are: (1) Reef talus or backreef breccia produced by storm wave action. The stratigraphy, clast-type, and orientation rule against this hypothesis. (2) Weathering breccia where joints, opened by solution, later were buried deeply and underwent collapse. This hypothesis, proposed by Taylor, is rejected by Parker because of lack of evidence of an erosion surface and lack of a distinct preferred orientation in the vein pattern. (3) Gypsum-anhydrite hydration and dehydration, plus solution hypothesis. Long-term episodes of hydration, dehydration, and solution of thin lenses of evaporites have brecciated the limestone layer. Park r believes that the gypsum-anhydrite lenses originally were interbedded with the limestone as products of normal evaporite sedimentation during a low sea level in the Late Mississippian. Frosted quartz grains in the brecciated limestone, as well as in the underlying nonbrecciated limestone suggest the presence of nearby land, perhaps arid. Actual evidence of any evaporite lenses are lacking at Bellevue and constitute Taylor's main objection to this theory.

A last stage of groundwater solution and precipitation of pyrite and calcite cemented the breccia.

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